The invention relates to a process for smelting metallic raw materials in a shaft furnace, in which coke is burnt with preheated air and largely pure oxygen and the flue gases heat the metallic charge in countercurrent, and in which the melt is superheated and carburized in the coke bed.
Metallic and non-metallic materials, such as iron and non-ferrous metals, basalt and greenstone, are still smelted in coke-heated shaft furnaces in spite of the development of electrical and flame-heated smelting processes. Thus, about 60% of all iron materials are nowadays still produced in cupola furnaces.
The reason for this high market share of the cupola furnace is the continuous further development, with the development of the hot-blast cupola furnace and the use of oxygen amongst the large number of known process modifications being of importance.
Thus, for example, the process engineering disadvantages and metallurgical disadvantages of the cold-blast cupola furnace, such as
low iron temperatures PA1 high burn-off of silicon PA1 low carburization PA1 high coke consumption PA1 high sulfur absorption PA1 high wear of refractories PA1 the smelting output PA1 the iron temperature PA1 the coke charge PA1 lower carburization PA1 increase in the Si burn-off PA1 increase in the FeO content in the slag PA1 wall channeling in the furnace due to a reduction in the blast velocity
have largely been compensated by the development of the hot-blast cupola furnace.
Similar improvements are achieved by the use of oxygen, the oxygen being blown into the cupola furnace either by enriching the cupola furnace blast up to a maximum of 25% or by direct injection at subsonic velocity. Owing to the high operating costs, however, oxygen is employed only discontinuously, for example for rapid starting of the cold furnace or for raising the iron temperature for a limited period. The possibility of increasing the output, i.e. continuous use of oxygen, is exploited only in exceptional cases.
In spite of the introduction of these process modifications, it is still possible for
to be varied only within a very narrow range at the optimum operating point.
The relationship between melting output and blast rate as well as the rate of addition of oxygen is described by the known Jungbluth equation. This equation results from a generation of mass and energy, with the coke charge and the combustion ratio having to be determined empirically for every cupola furnace.
Linking the active parameters, namely blast rate, coke charge and combustion ratio, to the target parameters results in the smelting output diagram, FIG. 1, with curves of equal coke charge and equal blast rate.
This smelting output diagram, known as the Jungbluth diagram,must be determined empirically for every cupola furnace. A transfer to other cupola furnaces is not possible, since the operating behavior changes immediately when the conditions such as lumpiness of the coke, reactivity of the coke, charge composition, blast velocity, furnace pressure, temperature etc. are altered.
The heat losses are lowest at the temperature maximum. At unduly high blast rates, i.e. high flow velocity, the furnace is overblown. At unduly small air rates, i.e. unduly low flow velocity, the furnace is underblown. In both cases, the combustion temperature is lowered, since, on the one hand, the additional N.sub.2 ballast must also be heated and, on the other hand, heat is removed by the additional formation of CO. Furthermore, the elements accompanying the iron are more thoroughly oxidized in overblowing.
By using oxygen up to, for example, 24% by volume in the blast, the net line is shifted towards the top right, i.e. to higher temperatures and to higher iron throughputs. The temperature maximum flattens, and the furnace becomes insensitive to underblowing or overblowing.
A reduction in the coke charge at constant iron throughputs and reduced blast rate is not possible even with continuous addition of oxygen, since the iron temperature then falls and additional metallurgical and process engineering problems, such as
arise. The cupola furnace produces an iron which cannot be cast.
Since, from the point of view of combustion technology, a large excess of coke is present, a reduction in the quantity of coke at constant smelting output is of great interest for reason of economics, since the manufacturing costs of molten iron are affected essentially by the remelting costs and the raw material costs.
Furthermore, it has been known for a long time that, especially in the case of cupola furnaces having large frame diameters, the so-called "dead man" remains standing in the center of the furnace in spite of oxygen enrichment of the blast and/or direct oxygen injection at subsonic velocity. The reaction between the oxygen blown in and the carbon takes place only within a restricted region in the vicinity of the blast nozzle, i.e. the furnace operates with wall channeling.
The coke present in the center of the furnace does not contribute to the reaction, since, due to the low momentum, the combustion air cannot penetrate the bed located in front. The reaction zone is located in the immediate vicinity of the blast nozzle (FIG. 2a). The depth of penetration is not substantially increased by the known enriching of the furnace blast with oxygen or by blowing the oxygen in at subsonic velocity. Due to the higher availability of oxygen, the reaction zone is widened upwards owing to the pressure conditions (FIG. 2b).
As a precondition for the desired reduction in the quantity of combustion coke, uniform combustion across the furnace cross-section, i.e. uniform distribution of the available oxygen, must be the objective. For this purpose, the momentum, i.e. the velocity of the air or of the oxygen jets, must be increased beyond the target values to be described as state of the art hitherto.
The patent application GB 2,018 295 describes a system by means of which the oxygen is blown in by means of Laval nozzles incorporated centrally into the blast nozzles, i.e. at supersonic velocity, in order to minimize the wear or the refractory lining. It was not possible to reduce the coke charge.